Animal Conservation (2004) 7, 435–443 C 2004 The Zoological Society of London. Printed in the United Kingdom DOI:10.1017/S1367943004001635
Aerial surveys and the potential biological removal technique
indicate that the Torres Strait dugong ﬁshery is unsustainable
Helene Marsh1, *, Ivan R. Lawler1 , Donna Kwan1 , Steve Delean1 , Kenneth Pollock2 and Matthew Alldredge2
1 School of Tropical Environment Studies and Geography, James Cook University, Douglas, Townsville, 4811 and CRC Torres Strait, P.O. Box 772
Townsville 4801, Australia
2 Statistics, Biomathematics, and Zoology, North Carolina State University, Raleigh, NC 27695-8203, USA
(Received 23 December 2003; accepted 29 April 2004)
The globally signiﬁcant dugong population of Torres Strait supports an important indigenous ﬁshery for meat
and oil. The ﬁshery is protected by the Torres Strait Treaty between Australia and Papua New Guinea. A time
series of aerial survey estimates from 1987–2001 conﬁrms that there is considerable temporal variability in
the size of the dugong population in the region and adds to a growing body of evidence from other aerial
surveys and satellite tracking that dugongs undertake large-scale movements associated with temporal and
spatial changes in the distribution of their seagrass food. The magnitude of these effects on both the size
of the population and the catch cannot be disaggregated from the effects of population depletion from over-
harvesting. The Potential Biological Removal method was used in conjunction with the aerial survey data to
estimate sustainable anthropogenic mortality from all causes for a range of empirically-derived estimates of
dugong life-history parameters. These estimates of a sustainable harvest are so far below the current harvest
that it must be unsustainable. Governments should heed the Islanders’ requests for assistance in implementing
co-management of the ﬁshery as a matter of urgency.
INTRODUCTION stakeholders and consequently managers must operate in
a difﬁcult and polarised environment.
Chapter 6 of Agenda 21 explicitly recognises the need
The dugong, Dugong dugon, is a species of charismatic
for developing institutional arrangements that empower
megafauna, which is of cultural value to coastal
indigenous peoples, strengthen their participation in
Aborigines and Torres Strait Islanders in northern
natural resource management and ensure that their use
Australia (Johannes & MacFarlane, 1991). Dugongs have
of resources is ecologically sustainable. The challenges
been hunted in the region for thousands of years. Their
in developing such arrangements are potentially greatest
meat is ranked highest among traditional foods. Hunting
when natural resources include species that are listed as
practices and prowess represent important aspects of
threatened, especially if they are of cultural and dietary
Aboriginal and Islander traditions.
value to indigenous peoples. Particularly in developed
As the only surviving member of the family Dugongidae
countries, the wider community often perceives indigen-
(Marsh et al., 1999), the dugong is of high biodiversity
ous hunting as a major threat to wildlife, particularly
value and is listed as vulnerable to extinction at a global
charismatic megafauna, even when there is little scientiﬁc
scale by the IUCN (Hilton-Taylor, 2000). Anecdotal
basis for this perception (Bomford & Caughley, 1996;
evidence suggests that dugong numbers have decreased
Freeman & Bogoslovskaya, 1998). Indigenous people
throughout most of their range (Marsh, Penrose & Eros,
perceive that they are competing with conservationists
2003), but that signiﬁcant populations persist in Australian
for wildlife and are concerned about their future access to
waters. Australia has obligations to protect dugongs as a
prey species and the survival of important components
signatory to various international conventions including
of their culture. Conservationists and researchers are
the Convention of Biological Diversity. It also has
concerned about over-exploitation, potentially leading to
obligations under that Convention to protect traditional
local extinctions. Policy makers and day-to-day managers
culture. In most coastal regions in tropical Australia,
have statutory responsibilities to conserve threatened
dugong hunting is a signiﬁcant component of traditional
species. There is resultant tension among all these
Dugongs occur along much of the tropical and sub-
tropical coast of Australia from Shark Bay in Western
∗ Australia to Moreton Bay in Queensland. The impacts
All correspondence to: Helene Marsh. Tel: 61-747-815575;
Fax: 61-747-816126; E-mail: email@example.com on the dugong include habitat loss and degradation,
436 H. MARSH ET AL.
pollution, incidental drowning in commercial gill nets and METHODS
shark nets set for bather protection, boat strike, illegal
poaching and indigenous hunting. The effects of these
impacts are difﬁcult to disaggregate and their relative All surveys used the aerial survey technique detailed in
importance varies in different areas (Marsh et al., 2002, Marsh & Sinclair (1989) and essentially the same survey
2003). Throughout most of the remote regions of design (Fig. 1) and were performed in November and/or
northern Australia north of about 16◦ S, the major impacts December in 1987, 1991,1996 and 2001. In 1996 and
are traditional hunting and accidental drowning in gill 2001 only, we used two aircraft ﬂying concurrently with
nets. separate teams of observers over periods of less than a
The globally signiﬁcant dugong population in the week, enabling the survey to be conducted under good
remote region of Torres Stait between Cape York and conditions while minimising the likelihood of dugong
Papua New Guinea (see Fig. 1) supports an important movements between survey blocks within the survey
traditional ﬁshery undertaken by Torres Strait Islanders period. The total survey area was 30 560 km2 . The overall
for meat and oil. On the basis of wet-weight landings, the sampling intensity was 5.85%, ranging from about 4% in
dugong ﬁshery is the largest island-based ﬁshery in the Block 3 to 10% in Block 5 (Fig. 1).
Torres Strait Protected Zone, which was established by
the Torres Strait Treaty between Australia and Papua New
Guinea to protect the Islanders’ traditional way of life and Estimating the size of the dugong population
livelihood. Dugong hunting is integral to this customary
way of life or Ailan Kastom and is currently ‘virtually Estimates of dugong abundance were obtained using two
unrestricted’ (Waia, 2001), despite the declaration of a methods:
dugong sanctuary where hunting was technically banned
1. Marsh & Sinclair (1989), which provides standardised
in the western Torres Strait in the1980s (see Fig. 1).
relative estimates of dugong abundance (all surveys);
The sustainability of this ﬁshery is a major imperative
2. Pollock et al. (2004, unpublished results), which
for the Torres Strait peoples who greatly value dugongs for
provides an absolute estimate of dugong abundance
their nutritional, cultural, social, economic and ideological
on which to base the estimate of the sustainability of
signiﬁcance. The issue is also a priority for managers in
the catch (2001 survey only).
relevant government environment agencies, particularly
the Australian Fisheries Management Authority (AFMA). Both methods corrected for the following survey
Scientists have voiced their concern about the sustain- biases: availability bias (animals not available to observers
ability of this ﬁshery since the mid-1980s (Hudson, because of water turbidity) and perception bias (animals
1986; Johannes & MacFarlane 1991; Marsh, 1996; Marsh, visible in the survey transect but missed by observers
Harris & Lawler, 1997; Marsh et al., 2002) but it has (sensu Marsh & Sinclair, 1989)). The method of Pollock
been difﬁcult to obtain the data required for a robust et al. (2004, unpublished results) should be more accurate
scientiﬁc assessment. The Australian and Queensland than that of Marsh & Sinclair (1989) because the
governments has been reluctant to intervene in the corrections for availability bias are based on empirical
management of this ﬁshery without ﬁrm evidence that data obtained from: (1) experiments to determine zones of
the dugong is over-harvested, because such intervention detectability for dugongs over a range of depths, turbidities
would be controversial in the difﬁcult political climate that and sea states using ﬁberglass models of dugongs as ‘sechi
characterises indigenous affairs in Australia. disks’ and (2) dive proﬁles obtained from time depth
Much of the information used to manage dugong recorders on 15 wild dugongs, enabling the probability
populations in Australia has been provided by aerial of a dugong being available to be estimated for different
surveys using a standardised technique developed by depths, water turbidities and sea states.
Marsh & Sinclair (1989). The Torres Strait region was Dugong abundance was estimated separately for each
surveyed in 1987, 1991, 1996 and 2001 to provide an block in the survey area (Fig. 1) using the Ratio Method
assessment of the distribution and abundance of the (Jolly, 1969; Caughley & Grigg, 1981). Input data were the
dugong in Torres Strait and a time series for temporal corrected number of dugongs (in groups of <10 animals)
comparisons. We present an assessment of the sustain- for each side of the aircraft per transect. The standard
ability of the dugong harvest in Torres Strait based on the error estimates incorporated the errors associated with all
results of this series of surveys, plus information on the the correction factors described above. Any dugongs in
size of the indigenous harvest relative to an estimate of a groups of >10 were added to the estimates of population
sustainable harvest obtained using the Potential Biological size and density as outlined in Norton-Grifﬁths (1978).
Removal method (Wade, 1998). The results indicated All population estimates are given ± standard error.
that, although there is considerable temporal variability
in the size of the dugong population in Torres Strait, it
is likely that the present indigenous harvest is an order
of magnitude too high to be sustainable. In a companion
paper (Heinsohn et al., 2004) reach a similar conclusion Differences in dugong density between survey years in
using Population Viability Analysis. Torres Strait were examined using a split-plot analysis of
Aerial surveys and the Torres Strait dugong ﬁshery 437
0 1B 1A
301 202 215
302 204 217
205 2A 218 2B
303 206 219
304 210 223
3 211 224
308 4 Survey blocks
404 Survey transects
311 405 Dugong Sanctuary
314 409 502
315 411 504 5
412 5041 505 20 0 20 40 60 kilometres
Fig. 1. Map of the survey region showing the positions of the survey blocks and transects and the Dugong Sanctuary established in the
1980s but never effectively enforced.
variance using the data generated by the method described in the Torres Strait region. The PBR is deﬁned as
by Marsh & Sinclair (1989). Mixed-effects models were the maximum number of animals, not including natural
employed to estimate the random components of variance mortalities, that may be removed from a marine mammal
for this analysis and to provide appropriate tests for stock while allowing that stock to reach or maintain
differences between years. The parameters of these models its optimum sustainable population, which is deﬁned as
were estimated using restricted maximum likelihood a population level between carrying capacity and the
(REML). Variation in dugong density between blocks population size at maximum net productivity. Thus, the
and between transects within blocks were random sources speciﬁc goal of the PBR is to allow each stock to reach or
of variance, as was the variation due to the interaction maintain a level at or above the maximum net productivity
between blocks across years. The (ﬁxed) year effect was level (MNPL: Wade, 1998). The PBR is calculated using
tested against the (random) block*year interaction using the following formula:
dugong density in each transect within blocks as the
PBR = Nmin × 0.5 Rmax × RF (Wade, 1998)
response. The data were log transformed (i.e. ln (y + 0.1))
to ensure a constant mean-variance relationship. The The minimum population estimate of the stock, Nmin , is
test for the year effect assumed sphericity (i.e. constant deﬁned as the 20th percentile of a log-normal distribution
correlation between blocks across years) and conservative based on an absolute estimate of the number of animals, N,
tests were performed in case this assumption was violated. in that stock. We used the method of Pollock et al. (2004,
Beaufort Sea State was included as a single degree of unpublished results) to generate the absolute estimate N
freedom covariate in the analysis. The term estimating the for the 2001 survey.
linear association of Beaufort Sea State with density was Rmax is the maximum rate of increase and 0.5 Rmax is
conditional on the other terms in the model. a conservative surrogate for RMNPL because 0.5 Rmax will
always be < RMNPL if MNPL is ≥ the carrying capacity
(Wade, 1998). The estimates of Rmax for Torres Strait are
based on empirical estimates of age of ﬁrst reproduction
Estimating the size of a sustainable dugong catch
and fecundity obtained by Boyd, Lockyer & Marsh (1999)
The Potential Biological Removal (PBR) Method (Wade, and Kwan (2002) and a pattern of natural mortality based
1998) was used to estimate a sustainable dugong catch on that obtained from longitudinal studies of manatees
438 H. MARSH ET AL.
Table 1. Leslie Matrix estimates of the maximum rate of population increase Rmax for dugong populations for combinations of life-history
parameters (age of ﬁrst calving and mean calving intervals spanning the known range of these parameters in various wild populations)
Age at ﬁrst
Rmax 2,3 for each of the following mean calving intervals
(years)1 2.5 years1 3 years 5 years4
Mabuiag, Torres 6 5.08% 3.9% 1.15%
Townsville 1970-early 10 3.35% 2.45% 0.3%
Daru, Torres Strait 13 2.46% 1.65% − 0.22%
Mornington Island 15 1.92% 1.2% − 0.53%
These data are based on recorded age of ﬁrst reproduction (calf birth) rather than mean age of ﬁrst reproduction and are taken from Boyd
et al. (1999) and Kwan (2002).
The population models use survivorship schedules based on empirical data for the Florida manatee as follows: dependent calves = 0.822
p.a.; independent young = 0.965 p.a.; reproductive adult = 0.965 p.a. (see Boyd et al., 1999 for details).
The age distribution has been truncated at 45 years. Extending it to the maximum age recorded for dugongs of 70 years makes only a
Heinsohn et al. (2004) also use a mean calving interval of 6.3 years but show, using Population Viability Analysis, that this was unlikely
in Torres Strait during the period spanned by the aerial surveys.
(Eberhardt & O’Shea, 1995; Langtimm et al., 1998). In different from that in 1987 (13319 ± 2136) but signi-
view of the uncertainty associated with these estimates, we ﬁcantly lower than the corresponding estimates derived
used a range of estimates for Rmax of 0.01–0.05 (1–5%) for 1991 (24225 ± 3276) and 1996 (27881 ± 3095:
as explained in Table 1. Tables 3 and 4, Fig. 2). Contrasts suggested signiﬁcant
The recovery factor (RF) of < 1 allocates a proportion increases between 1987 and 1991 (contrast estimate =
of expected net production towards population growth 0.305, p = 0.024) and between 1991 and 1996 (contrast
and compensates for uncertainties that might prevent estimate = 0.339, p = 0.012) and a signiﬁcant decline
population recovery, such as biases in the estimation of between 1996 and 2001 (contrast estimate = − 0.667,
Nmin , and Rmax or errors in the determination of stock p < 0.001). There was no difference between the estimates
structure. Population simulations (Wade, 1998) suggest
that the default value for stocks of unknown status such as
the dugong population of Torres Strait should be 0.5, the
value used here. The suitability of this value rather than
an alternative is considered in the discussion below. 0.6
Dugong density (animals per km2 ± 95% CI)
We obtained dugong catch estimates from various parts 0.4
of Torres Strait from the literature as detailed in Table 2.
The estimates include ‘guesstimates’, periodic sampling
surveys and catch censuses. The values reﬂect differences 0.3
in monitoring technique as well as spatial and temporal
variation. The most accurate records are those of Kwan 0.2
(2002) who lived at Mabuiag Island for 9 months in each
of 1998 and 1999 and recorded carcasses as they were
butchered at traditional sites. 0.1
1987 1991 1996 2001
Aerial survey estimates of dugong density Year of survey
and population size
Fig. 2. Estimated mean dugong density (± 95% conﬁdence interval)
Using the method of Marsh & Sinclair (1989), the for each of the four surveys conducted in Torres Strait. The data
estimated size of the dugong population in Torres Strait for all surveys have been generated using the method described by
in 2001 was 14061 ± 2314, which is not signiﬁcantly Marsh & Sinclair (1989). CI, conﬁdence interval.
Aerial surveys and the Torres Strait dugong ﬁshery 439
Table 2. Estimates of the catch of dugongs in various Torres Strait communities between the 1970s and the 1990s
Estimated dugong catch
Method of estimating (annual unless otherwise
Area catch Date indicated) References
Mabuiag Is. Sporadic census 1973 24 Bertram & Bertram (1973)
Census 1977 103 Nietschmann (1985)
Sporadic census 1983 1984 12 Johannes & MacFarlane (1991)
Survey 274 (SE 175)
Census (9 mths) 1994 145 Kwan (2002)
Census (9 mths) 1998 170 Kwan (2002)
Survey 1999 183 (SE 77)
Badu Is. Survey 1994 107 (SE 80)
Survey 1999 200 (SE 66)
Survey 2000 166 (SE 65)
Boigu Is. Survey 1994 256 (SE 110)
Survey 1999 128 (SE 59)
Survey 2000 87 (SE 26)
TSPZ1 Census 1976–77 750 Nietschmann (1985)
Sporadic census mid-1980s 110–130 Johannes & MacFarlane (1991)
Census 1991–92 954
Survey 1991–92 1010 (SE 240)
Survey 1991–93 1226 (SE 204)
Survey 1994 860 (SE 241)
Survey 1996 241 (SE 92)3
Survey 1998 256 (SE 136)3
Survey 1999 692 (SE 150)
Survey 2000/01 619 (SE 134)
Bamaga Survey 1997 116
Daru PNG Census 1976–772 74–120 Hudson (1986)
Census 1978–832 463 Hudson (1986)
All of the survey results were obtained by the CSIRO and the Australian Fisheries Management Authority. SE, standard error; TSPZ,
Torres Strait Protected Zone; PNG, Papua New Guinea.
Includes total catch for the Torres Strait Protected Zone (TSPZ) including Mabuiag, Badu and Boigu Islands.
Catch statistics recorded during the period when dugong meat was legally sold in the Daru market.
Rejected by Torres Strait Fisheries Scientiﬁc Committee because of low precision and potential downward bias.
Table 3. Estimates of dugong abundance (± standard error (SE)) in each of the survey blocks in Torres Strait (Figure 1) for surveys
conducted in 1987, 1991, 1996 and 2001
2001 Marsh & 2001 Pollock
1987 1991 1996 Sinclair method et al., method
Block Mean SE Mean SE Mean SE Mean SE Mean SE
0 0 696 238 1152 381 0 0 0 0
1A 1131 278 1669 999 2427 663 685 317 635 94
1B 3705 1529 1681 615 2678 1695 1757 475
2A 6424 1679 9113 1798 10869 1600 3504 403 3429 453
2B 2019 573 1467 399 1905 370 583 166 440 83
3 2822 1102 6740 1958 8623 2411 5473 1327 4927 972
4 848 347 518 197 984 313 1183 655 778 150
5 76 55 320 277 240 70 0 0 0 0
Total 13319 2136 24225 3276 27881 4720 14106 2314 11956 1189
of dugong density in 2001 and 1987. The between- (0.02) was very small suggesting that these spatial
blocks variance component (0.375) was large relative differences are temporally robust (Fig. 3). The largest
to the variance between transects within blocks (0.171) variance component (0.756) corresponds to the between-
suggesting that some parts of the region (e.g. especially transect within block variation between years (error)
Block 2A, Fig. 1) are consistently much more important to suggesting that dugongs make substantial small-scale
dugongs than others. The year*block variance component movements within blocks over time.
440 H. MARSH ET AL.
Table 4. Results of the split-plot analysis of variance examining Estimating a sustainable level of human-induced
dugong density between surveys mortality for dugongs in the Torres Strait survey region
Term SS d.f. MS Est. Var. F p The range of estimates for sustainable anthropogenic
mortality (PBR) are summarised in Table 5 for the
Block3 154.4 7 22.05 0.375 1996 and 2001 estimates of dugong population size. The
Transect (Block)3 124.3 86 1.45 0.171 middle value for the estimated maximum rate of incre-
Year1,2,4 25.8 3 8.61 8.89 0.0005 ase R max (= 0.03) and an RF value of 0.5 suggest that the
Year*Block3 20.4 21 0.97 0.020 1.28 0.187
following total annual anthropogenic mortalities should
Beaufort2 2.3 1 2.34 3.10 0.080
Error 194.3 257 0.76 0.756
be sustainable: about 190 dugongs for the1996 estimate;
about 90 dugongs for the 2001 estimate using the method
The estimated variances (Est. Var.) are calculated from the mixed- of Marsh & Sinclair (1989). The 2001 ﬁgure using the
effects analysis. SS, sum of squares; MS, mean square. method of Pollock et al. (unpublished) is 82 dugongs.
Tested against year by block interaction.
Fixed factor covariate.
Random factor. DISCUSSION
Conservative lower bound test for year effects, which does not
assume sphericity: F = 8.89, d.f. = 1 & 7, p = 0.021. Temporal changes in dugong population size
in Torres Strait
The population estimates obtained from the survey
The population estimate for 2001 using the method conducted in November 2001 conﬁrm that there is
of Pollock et al. (2004, unpublished results) was 11915 considerable temporal variability in the estimated size
(± 1198) dugongs, about 15% less than the corres- of the dugong population in Torres Strait (Table 3).
ponding estimate using the older methodology. Marsh et al. (1997) reviewed the possible reasons for
Table 5. Estimates of the total sustainable anthropogenic mortality (Potential Biological Removal sensu Wade, 1998) for the 1996 and
2001 aerial survey estimates of the size of the dugong population in Torres Strait for a range of estimates of Rmax and assuming an RF
value of 0.5
Potential Biological Removal
Date of survey R.F. N SE CV Nmin Rmax = 0.01 Rmax = 0.02 Rmax = 0.03 Rmax = 0.04 Rmax = 0.05
2001 0.5 14106 2314 16.4 12297 31 61 92 123 154
1996 0.5 27881 3095 11.1 25400 64 127 191 254 318
The values for the PBR based on the 2001 survey estimate are based on the population estimate derived using the method of Marsh &
Sinclair (1989). The values derived using the method of Pollock et al. (unpublished) are slightly lower (27–137).
1987 1991 1996 2001 1987 1991 1996 2001
Block 2B Block 3 Block 4 Block 5
Block 0 Block 1A Block 1B Block 2A
1987 1991 1996 2001 1987 1991 1996 2001
Fig. 3. Estimated mean dugong density (± 95% conﬁdence interval) in each block in each survey year. The data for all surveys have been
generated using the method described by Marsh & Sinclair (1989).
Aerial surveys and the Torres Strait dugong ﬁshery 441
the signiﬁcant difference between the dugong population or more (Poiner & Peterken, 1996). Halophila ovalis,
estimates obtained in 1987 and 1991 and concluded that one of the preferred food species of dugongs, appears
it: (1) could not be explained by natural increase in the to be particularly sensitive to light reduction, with the
absence of immigration into the survey area and (2) was duration and frequency of light-deprivation events being
unlikely to be the result of uncorrected ﬂuctuations in the the primary factors affecting its survival (Longstaff et al.,
availability bias between surveys, but (3) the possibility 1999). Members of the genus Halophila occur at greater
of double counting as result of dugongs moving between depths than other species of tropical seagrasses and this
survey blocks during the survey could not be dismissed. sensitivity to light reduction is a plausible contributor to
However, these last two explanations can now be ruled the causes of the large-scale loss of deep-water seagrasses
out. The empirical measurements of availability bias by in Torres Strait (Poiner & Peterken, 1996) and Hervey Bay
Pollock et al. (2004, unpublished results) indicate that in Queensland, Australia (Preen et al., 1995).
the method used to correct for availability bias in the Dugongs respond in one of two ways to large-scale
earlier surveys was robust and the use of two aircraft seagrass loss. Some animals remain in the area losing body
enabled the 1996 and 2001 surveys to be completed in a condition and delaying breeding; others move hundreds
few days. Marsh & Lawler (2001, 2002) and Gales et al. of kilometres with varying probabilities of survival. For
(2004) also explored possible reasons for the temporal example, after the loss of seagrass in Hervey Bay in 1992,
differences between population estimates obtained from the estimated size of the dugong population in that region
repeat aerial surveys of the southern Great Barrier Reef declined from 2206 ± 420 in 1988 to between 521 and 571
and Hervey Bay, the northern Great Barrier Reef and with a standard error of 126 in 1993. Ninety-nine dugong
Western Australia, respectively, and concluded that large- carcasses were recovered in the Hervey Bay area, on the
scale movements of dugongs were the most likely expla- central and southern Queensland coast and along the New
nation, a conclusion which we support. This pattern is South Wales coast (Preen & Marsh, 1995). Most animals
also consistent with traditional knowledge (Johannes & appeared to have died of starvation. The percentage of the
MacFarlane, 1991), anecdotal information and satellite population remaining in Hervey Bay that was identiﬁed
tracking (Marsh & Rathbun, 1990; Marsh et al., 2002, as calves on the basis of aerial surveys plummeted from
2003), all of which suggest that dugongs undertake large- 22.5% in 1998 to 2.2% in 1993 and 1.55% in 1994 (Marsh
scale and/or long-distance movements. The satellite track- et al., 1996). The seagrass had recovered by late 1998
ing studies indicated that a dugong could move hundreds (Coles, McKenzie & Campbell, 2003) and the dugong
of kilometres in a few days but that such movements population estimated by aerial survey had increased to
are individualistic and not coordinated among dugongs. 1473 ± 242 by 1999 (Marsh & Lawler, 2001). As such
an increase is biologically impossible in the absence of
migration, we assume that dugongs migrated to the region
Reasons for temporal changes in the size of the dugong as the seagrass recovered.
population of Torres Strait: the role of seagrass dieback The response of dugongs to seagrass loss seems similar
in Torres Strait. Islanders interviewed by Johannes &
Large-scale dugong movements appear to be a response MacFarlane (1991) were unanimous that an unusually
to seagrass dieback. Episodic losses of hundreds of high proportion of dugongs caught in Torres Strait during
square kilometres of seagrass are associated with extreme the 1970s were lethargic, thin and poor-tasting ‘wati
weather events such as some cyclones, hurricanes and dangal’. Niestchmann (1985) reported an unusually high
ﬂoods (Poiner & Peterken, 1996). Such losses have proportion of algae and the seagrasses Thalassia and
been recorded in Torres Strait and in other regions of Enhalus in the stomach contents of dugongs caught in
northern Australia. Islanders observed a major dieback of 1976–1977. Spain & Heinsohn (1973) also report dugongs
seagrasses in waters extending from Badu Island south eating unusually high proportions of algae, Thalassia and
to Thursday Island in the early 1970s. The cause of the Cymodocea when seagrass was in short supply after a
dieback was never conﬁrmed (see Johannes & McFarlane, cyclone near Townsville in 1971. Dugongs do not appear
1991 for a more complete account of this controversy). In to be well adapted to using algae as a food source and
1991–1992, several hundred square kilometres of seagrass apparently prefer seagrass species such as Halophila and
disappeared from northern Torres Strait, probably because Halodule rather than Thalassia, Cymodocea and Enhalus
of high turbidities resulting from ﬂooding of river(s) in (Marsh et al., 1982).
Papua New Guinea coincident with an El Ni˜ o Southern Hudson (1986) presented anecdotal evidence that none
Oscillation (ENSO) event (Poiner & Peterkin, 1996). of the 35 female dugongs landed at Daru between
Local ﬁshers again reported seagrass dieback in the November 1976 and July 1997 was pregnant. Kwan (2002)
Orman Reef area (the most important dugong habitat in reported a corresponding gap in the age distribution of
Torres Strait) in 1999–2000, prior to our last survey. dugongs caught at Mabuiag in 1997–1998 conﬁrming
Extreme weather events cause extensive damage to that recruitment failure was widespread in dugongs in
tropical seagrass communities through severe wave action, Torres Strait at the time of the 1970s seagrass dieback. In
shifting sand, adverse changes in salinity and light addition, Marsh (1995a) reported a monotonic increase
reduction (Heinsohn & Spain, 1974; Preen & Marsh, in the pregnancy rate of dugongs landed in Daru
1995; Preen, Lee Long & Coles, 1995). Recovery and between 1978 and 1981, the period of reported seagrass
recolonisation after such losses may take up to a decade recovery.
442 H. MARSH ET AL.
Given the anecdotal reports of seagrass dieback in the that is impacted on by occasional stochastic events is
Orman Reef area (the most important dugong habitat in questionable. The use of a more conservative value such
Torres Strait) in 1999–2000, it is salient that the dugong as 0.1 would mean that the harvest rate would be very
population in Block 2A was signiﬁcantly lower during low (38 dugongs per annum for the entire region based
the 2000 survey than during the 1991 and 1996 surveys on the 1996 abundance estimate; 16–18 dugongs per
(Fig. 3). In addition, the percentage of calves in the region annum based on the 2001 estimate) or that the ﬁshery
of the reported seagrass dieback around Orman Reef plus would have to be closed. In our opinion even though
a region where we observed exceptionally turbid water conservation and animal rights groups would support
during our survey (Blocks 0, 1A, 1B, 2A and 2b) was such an approach, setting such a low total allowable
only 5.2%. This was signiﬁcantly lower than the 19.2% catch is likely to be a major obstacle to progressing
reported from the region to the west (Block 3) and south co-management arrangements with indigenous groups
(Block 4) where no seagrass dieback was reported. (Yates’ throughout this remote region. Such action may also
corrected χ 2 = 13.19, d.f . = 1, p = 0.0003). require the renegotiation of the Torres Strait Treaty
Unfortunately the magnitude of the effects of emigra- between Australia and Papua New Guinea.
tion or immigration on the size of a dugong population Some Australian Torres Strait Islander leaders have
cannot be disaggregated from the effects of population called for government assistance with implementing co-
depletion from over-harvesting. Thus the recent trends management arrangements for hunting for many years
detected by aerial surveys (Fig. 2) are not a reliable index (e.g. Resolution from the workshop Towards Community-
of the status of the Torres Strait dugong population. Based Management of Dugongs and Turtles in Torres
Even if they were, relying on population trends to Strait held by AFMA in 1998). Scientists (e.g. see
trigger management actions is likely to result in Type 2 Johannes & MacFarlane, 1991; Marsh, 1996; Marsh et al.,
error since trends are not likely to be detected until a 1997) have made similar demands, but there has been
population is seriously depleted (Taylor & Gerrodette, little progress. In 2003, the Australian Minister of
1993; Marsh 1995b). Environment instructed that this matter be addressed
with high priority and management agency staff are
working with traditional owners to develop a mutually
Preliminary estimates of a sustainable harvest
acceptable framework for sustainable dugong hunting. In
Evaluating whether or not the population can support addition, considerable funding has recently been identiﬁed
the known level of harvesting should have a lower risk for community-based management and catch monitoring
of Type 2 error (Wade, 1998) despite the challenge initiatives.
of estimating absolute abundance. Pollock et al. (2004,
unpublished results) compared estimates of dugong abun-
dance obtained using their new technique with estimates
obtained using the technique of Marsh & Sinclair (1989)
for two aerial surveys: the 2001 survey of Torres Strait The evidence presented here that dugongs are over-
reported here and a survey of the northern Great Barrier harvested in Torres Strait indicates that the present
Reef region in 2000. In both cases the resultant population system of essentially unregulated dugong hunting is not
estimates were reasonably close (within −15% to + 7% sustainable. Heinsohn et al. (2004) came to a similar
of each other) suggesting that the estimates obtained from conclusion using Population Viability Analysis. Co-
the 1987, 1991 and 1996 surveys should also be close management arrangements for indigenous hunting in
to absolute estimates of dugong abundance in the survey Torres Strait must be progressed as a matter of urgency,
region at the times of those surveys. if Australia is to honour its international commitments to
Thus, based on the 1996 abundance estimate and conserve dugongs and dugong hunting cultures. Dugong
assuming an RF value of 0.5, we estimate that the hunting is a very signiﬁcant part of the Torres Strait
sustainable harvest is about 190 dugongs per annum Islanders’ traditional culture. This culture cannot be
for the entire region including the Papua New Guinea conserved if the target species becomes locally extinct.
communities, the Protected Zone, the Inner Australian
Islands and the Northern Cape York Peninsula Area
based on the 1996 abundance estimate. The corresponding
ﬁgure based on the 2001 population estimate is 80–
90 dugongs, an estimate similar to that independently The Australian Fisheries Management Authority funded
estimated by Heinsohn et al. (2004) using Population the Torres Strait surveys and their staff in Torres Strait
Viability Analysis. This estimate is substantially less than provided logistical support. We thank the following people
the harvest recorded between April and September 1998 for their invaluable assistance with the survey and/or
and 1999 from a single major hunting community – subsequent reporting: our observers and pilots; Torres
Mabuiag Island (Kwan, 2002) and considerably less than Strait Regional Authority; Chairmen and members of
the catch estimates for the various components of the communities of Horn Island, Mabuiag, Boigu, Kubin and
whole region (Table 2). Saibai; CRC Torres Strait; Adella Edwards for assistance
Even though it is technically correct (Wade, 1998), with ﬁgures; Pam Quayle and Jeanine Almany for editorial
the use of a RF value of 0.5 for a threatened species assistance.
Aerial surveys and the Torres Strait dugong ﬁshery 443
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